Ferroelectrics (FE) are electrically polarisable materials which possess spontaneous polarisation below the Curie temperature (TCE), and the polarisation in ferroelectric materials is switchable with respect to an external electric field. Similarly, ferromagnets have induced magnetisation below the Curie temperature (TCE) and the magnetisation in ferromagnetic materials is switchable with respect to an external magnetic field.
The field-driven switching of the ferroelectric and ferromagnetic properties forms the basis of ferroelectric random access memory (FERAM) and magnetic random access memory (MRAM) devices respectively [1-3]. Both devices are non-volatile and have certain advantages over conventional random access memory devices (RAMs).
FERAMs offers faster writing performance over conventional RAMs, whilst MRAMs offers non-destructive magnetic reading [4].
The full scale commercialisation of FERAMs and MRAMs, however, has been constrained by certain drawbacks suffered by these devices. FERAMs suffer from low storage densities, whereas MRAMs suffer from high writing energy consumption [4].
Materials that exhibit both a magnetisation and a dielectric polarisation in a single phase are referred to as multiferroic or magnetoelectric materials.
The induction of magnetic moment by an external electric field or polarisation by a magnetic field is known as the magnetoelectric effect and is a property of multiferroic materials. The use of the magnetoelectric effect has been proposed in many applications, such as magnetic field sensors [5,6], magnetoelectric MRAM (ME-MRAM) [7] and microwave devices [8].
Ferroelectricity and ferromagnetism (or antiferromagnetism) have different electronic structure requirements and normally do not coexist in single phase materials. Conventional mechanisms for ferroelectricity involve closed-shell d0 or s2 cations, whereas ferromagnetic order requires open-shell dn configurations with unpaired electrons [9]. This fundamental distinction has made it difficult to combine long-range order of the two dipoles to simultaneously break space inversion and time-reversal symmetry at room temperature [10].
However, there are design routes which can produce both orders in single phase materials such as the ABO3 perovskite BiFeO3. However, BiFeO3 is known to inhibit weak ferromagnetism and linear magnetoelectric coupling due to the cycloidal magnetic ordering, and consequently is not commercially useful.
In a multiferroic material with strong magnetoelectric coupling, the polarisation or magnetisation will be switchable with respect to magnetic field or electric field. Therefore, the shortcomings in FERAMs and MRAMs could be avoided by employing suitable multiferroic materials, such that low energy ferroelectric writing and non-destructive magnetic reading could be achieved [11].
Unfortunately, at present there has been no single phase bulk material reported that demonstrates long-range ordered switchable polarisation and magnetisation at room temperature.[12]
There therefore remains a need for new and improved single phase materials that exhibit improved magnetoelectric properties.
In particular, there is a need for new and improved single phase materials that exhibit a magnetoelectric effect over the typical operational temperature ranges of electronic devices (e.g. at room temperature).
In addition, there is a need for materials that can easily be formed into multiferroic thin films using thin-film deposition techniques known in the art. Such thin films can be incorporated into a wide variety of electronic components, such as, for example, MRAM, FERAM or MERAM components.
The present invention was devised with the foregoing in mind.